The invention relates to early fuel evaporation types of fuel supply systems using self-regulating ceramic resistance heaters and the invention relates more particularly to a system incorporating a heater having novel contact means which provide the system with improved service life characteristics when used with alcohol or gasohol fuels.
Electrical resistance heaters formed of ceramic materials of positive temperature coefficient of resistivity are known to sharply increase in resistivity as they are heated above the Curie temperatures of the materials and are known to self-regulate at predetermined temperatures to provide selected, safe, heating effects. That is, the heaters generate a substantial amount of heat when they are first energized but then display greater resistance and reduce heater current as they heat up, thereby to decrease their rate of heat generation until they stabilize at a temperature at which the rate of heat generation by the reduced current balances the rate of heat dissipation from the heaters. Such heaters have been arranged in automotive fuel supply systems to be energized when operation of the automotive engine is initiated as is shown in U.S. Pats. No. 3,987,772, No. 4,107,515, No. 4,180,901, No. 4,189,509, No. 4,189,700, No. 4,108,125 and No. 4,141,327. In those systems, the heaters provide a prompt heat output sufficient to enhance fuel evaporation to facilitate smooth engine starting on a cold day and to reduce the emission of pollutants in the engine exhaust during engine warm-up period. The heater stabilizes at a safe maximum temperature level below the fuel ignition temperature and reduces its rate of power consumption as engine warm-up occurs.
Many different types of coatings have been provided on selected portions of the bodies of such self-regulating ceramic heaters to serve as contacts or electrodes for applying electrical power to the ceramic resistance materials as is shown in "Ohmic Contacts to Semiconducting Ceramics" by D. R. Turner and H. A. Sauer, J. Electrochem. Soc. 107,240 (1960); U.S. Pat. No. 3,023,390; "Electrodes for Ceramic Barium Titanate Type Semiconductors" by Harry M. Landis, J. Applied Physics 36(6) pp. 2000-2001 (1965); U.S. Pat. No. 3,676,211; U.S. Pat. No. 3,927,300; U.S. Pat. No. 4,055,438; U.S. Pat. No. 4,032,752; U.S. Pat. No. 4,189,509; "Formation of Ohmic Contacts in Semiconducting Oxides" by J. Narayan and V. N. Shukla, J. Applied Physics 51(6), pp. 3444-3446 (1980); and U.S. Pat. No. 4,232,214. Some coatings form ohmic contacts which have low electrical resistance, which are non-rectifying, and which have the property that the potential difference across the contacts to the ceramic material is proportional to the current being applied to the ceramic material through the contacts in accordance with Ohm's law. Other coatings form higher resistance, non-ohmic contacts to the ceramic materials. Some known coatings embody expensive materials; some are difficult to apply to selected areas of the ceramic surface; some age or display increasing resistance as they are thermally cycled in engagement with the oxygen-rich ceramic materials so that they tend to provide varying heating effects; and some tend to lose their bond to the ceramic material during thermal cycling. Further, some coatings are suitable for use in certain high voltage applications but are considered unsuitable for use in other types of lower voltage systems. In this regard, known contact systems for such ceramic heaters do not appear suitable for successful, low cost commercial use in early fuel evaporation systems for automobiles where alcohol or gasohol fuels may be encountered. Such known contact systems appear to be either too expensive to use; too high in initial resistance properties; or too subject to change in resistance characteristics during use in an alcohol vapor atmosphere so that they tend to fail or to provide varying heating effects over the life of the automotive fuel supply system.
It is an object of this invention to provide a novel and improved self-regulating ceramic resistance heater; to provide such a heater which is particularly adapted for use in a alcohol vapor atmosphere; to provide such a heater which is adapted to display stable heating characteristics over a long service life when operated from a low voltage automotive power source in an early fuel evaporation system where alcohol or gasohol fuels may be used; and to provide a novel and improved automotive fuel supply system adapted for use with alcohol or gasohol fuels.